1. Field of the Invention
The present invention relates to a thermoelectric conversion material and a method for producing the thermoelectric conversion material, and more particularly, a thermoelectric conversion material which has a higher Seebeck coefficient, a lower electric resistivity, and a larger power factor than conventional composite oxides, and a method for producing such a novel thermoelectric conversion material.
2. Description of the Related Art
In recent years, attention has been focused on thermoelectric conversion elements (thermoelectric conversion modules) capable of converting heat energy directly to electrical energy, as an effective technique for utilizing waste heat.
As thermoelectric conversion materials for use in these thermoelectric conversion elements, materials which use the Seebeck effect have been widely known conventionally.
Further, materials with a high Seebeck coefficient (α) are desired because the voltage generated when a temperature difference is produced is preferably as high as possible in the case of the thermoelectric conversion materials (thermoelectric semiconductors).
In addition, the electric resistivity ρ is desirably smaller because the high electric resistance (electric resistivity ρ) in a current flow causes energy to be lost by Joule heat.
The property of a thermoelectric conversion material is determined by the index defined by the following formula (1), which is referred to as a power factor (P.F.).P.F.=α2/ρ  (1)
From this point of view, a thermoelectric semiconductor element has been proposed which uses an oxide ceramic semiconductor composed of a composite oxide containing strontium and titanium as its main constituents and including therein scattered reducing substance phases which are not continuous with each other (See, for example, claim 1 in Japanese Patent Application Laid-Open No. 5-129667).
It is to be noted that examples of the scattered reducing substance phases are reported to include metal phases containing titanium, zirconium, tantalum, niobium, or the like as their main constituent, or metal carbide phases.
Further, Japanese Patent Application Laid-Open No. 5-129667 shows thermoelectric conversion materials with a Seebeck coefficient (α) in the range of 120 μV/K to 197 μV/K and an electric conductivity in the range of 350/Ω·cm to 1010/Ω·cm (Table 1, and Table 2).
In this case, the power factor (P.F.) obtained for the thermoelectric conversion materials in Japanese Patent Application Laid-Open No. 5-129667 in accordance with the above formula (1) represents values of 5.8×10−4 (No. 5 in Table 2) to 2.3×10−3 W/K2m (No. 3 in Table 1), which are favorable properties at the time of filing. However, nowadays, thermoelectric conversion materials have been desired which have larger power factors.
In addition, as another thermoelectric conversion material, a thermoelectric conversion material has been proposed which is a composite oxide containing a strontium oxide and a titanium oxide as its main constituents, contains a rare-earth element and at least one or more elements selected from Nb, Ta, Sb, W, Si, Al, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, and has an electric conductivity of 100/Ω·cm or more (See, for example, claim 1 in Japanese Patent Application Laid-Open No. 8-236818).
The thermoelectric conversion material disclosed in Japanese Patent Application Laid-Open No. 8-236818 has a Seebeck coefficient in the range of −135 μV/K to −330 μV/K, and an electric conductivity in the range of 330/Ω·cm to 210/Ω·cm. The power factor (P.F.) obtained from these values in accordance with the above formula (1) represents values of 3.6×10−3 (No. 12 in Table 1 of Japanese Patent Application Laid-Open No. 8-236818) to 4.5×103 W/K2m (No. 3 in Table 1 of Japanese Patent Application Laid-Open No. 8-236818). While these values of the power factor are larger than those in Japanese Patent Application Laid-Open No. 5-129667 mentioned above, thermoelectric conversion materials with much larger power factors have been desired nowadays as a matter of fact.